1. Field of the Invention
The present invention relates to an image reading device, such as an image sensor, that converts optical information to an electric signal.
2. Description of the Prior Art
As FIG. 8 shows, an image reading device is composed of IC chips K1, K2, . . . , Km, each having photoelectric conversion elements formed thereon, arranged in a row and mounted on a printed circuit board (not shown). The IC chips K1, K2, . . . , Km sequentially output photoelectric conversion signals, which are then converted by an A/D converter 100 into a digital signal, which is then fed out via an output terminal 200. Conventionally, as the IC chips K1, K2, . . . , Km, IC chips of an identical configuration are used.
Within each IC chip, as the photoelectric conversion elements, a plurality of photodiodes PD1, PD2, . . . , PDn are formed in a row. FIGS. 9A and 9B are a top view and a sectional view, respectively, schematically showing such an IC chip. Here, a P-type substrate 21 serves as the anodes of the photodiodes PD1, PD2, . . . , PDn, and N-type regions 22 serve as the cathodes of the photodiodes PD1, PD2, . . . , PDn. On an insulating film 24, a metal conductor 25, such as one made of aluminum, is formed as a light shield film, with openings W left in positions corresponding to the N-type regions 22.
The conductor width W1 between two adjacent photodiodes is determined according to the resolution of the device as a whole. Moreover, to make the characteristics of the individual elements as uniform as possible, an identical pattern is formed repeatedly. As a result, the conductor width W2 on the chip edge 30 side of the photodiodes PD1 and PDn located at both ends of the IC chip is equal to about one half of the conductor width W1 between two adjacent photodiodes.
Furthermore, conventionally, to prevent short-circuiting between the substrate 21 and the conductor 25 during scribing, no conductor, even including the conductor 25 serving as the light shield film, is formed in the areas covering a predetermined distance W3, defined in design rules, from the chip edges 30.
Accordingly, conventionally, as the resolution of the device becomes higher, the conductor width W2 secured on the chip edge 30 side of the photodiodes PD1 and PDn located at both ends becomes narrower. This makes it more likely that unwanted light leaks in these photodiodes PD1 and PDn obliquely from above and through the sides of the IC chip as compared with the other photodiodes.
As described above, light shield conditions differ between for the photodiodes located at both ends and for the other photodiodes, and therefore, even if uniform light is shone on all the photodiodes, the cathode voltages of the photodiodes at both ends are usually significantly lower than the cathode voltages of the other photodiodes. This causes great variations among the signal levels obtained from the individual photodiodes.